Recovery of plutonium values from dilute solution by partial precipitation of carrier compounds



RECOVERY OF PLUTONIUM VALUES FROM DI-- LUTE SOLUTION BY PARTIAL PRECIPITATION' OF CARRIER CONIPOUNDS i? No Drawing. Application March 13, 1945 SerialNo. 582,592

Claims. c1; 23-145 The present invention relates to a method for theconcentration and recovery ofplutonium from solutions thereof and more particularly it concerns a procedure for the removal of that element from its solutions by carrier methods wherein the ratio of carrier employed to plutonium contained therein isreduced.

Naturally occurring uranium metal consists chiefly of three isotopes at least 99% of which is 92 about 0.7% 92 and a still smaller percentage of 92 It has been observed that uranium when irradiated with thermal neutrons by virtue of its 92 content forms another. isotope known as 92 which has a half life of about 23' minutes and which on beta decay is transformed into 93 now known as neptunium. The latter element possesses. a half life of 2.3 days and by .beta decay is converted into plu' tonium. Plutonium, when irradiated with neutrons, fissions to produce about 185 mev. per fission. In this connection the expression plutonium is intended to include either elemental or combined states of plutonium.

Likewise, it has beenfound that the 92 content of uranium, when subjected to irradiation with fast or thermal neutrons, fissions to produce a series of elements which can be divided into a light and a heavy element group, respectively, the former consisting of elements having atomic numbers ranging between about 35 and 46, and the latter consisting of elements of the heavy group having atomic numbers varying between 51 and 60. The formation of these elements is also accompanied by the release of relatively large quantitiesof energy mainly in the form of heat. These two element groups upon their immediate formation are commonly referred to as fission fragments. Such fragments when first formed are for the most part highly overmassed and undercharged, and hence, tend to be extremely radioactive. However, .nuclei thereof relatively rapidly transform themselves by radioactive decay into more stable isotopes and in such form are known as fission products. These products continue to possess varying degree of radioactivity, however, and in general must remain in a suitable storage Vault before they are biologically safe for working-personnel to handle.

Plutonium can be recovered from fission product mixtures of the above-mentioned type .in accordance with any of several procedures, the most common of which is based on the utilization of a carrier therefor. This particular method of separation is based upon the discovery that plutonium, when in avalence state not greater than 4, can be converted into a form which readily becomes associated with certain insoluble carriers and, as a result, is removed from solution. 'It has also been discovered that further separation of plutonium from fission product mixtures can be efiected by first converting'the plutonium to a valence greater than 4 in which state it is not readily carried by these particular carriers,-and then contacting the solution containing the same with one of the suitable carriers thereby removing the fission products with the carrier and leaving the plutonium together with a dc tates Patent Patented June 23, 1959 moval of plutonium from dilute fission product solutionsthereof will be foundin copending application Ser. No.

519,714, filed January 26, 1944, by Glenn T. Seaborg;

et 211., now US. Patent 2,785,951, issued March 19, 195.7. Methods based on the utilization of such oxidation-rednetion cycles, however, have not met with unqualified success. One of the principal disadvantages of procedures of the aforesaid type resides in the fact that during the oxidation and reduction of plutonium and during theremoval and washing of the carrier by-product precipitates, it is necessary to add relatively large volumes of the required reagents The addition so dilute the original carrier. solution that the volume thereof tends to approach. that of the solution from which the carrier was initially precipitated; Since the volume of solution governs theratio of carrier to plutonium contained therein, it is obvious that by the foregoing procedure no substantial reduction in such ratio will be eifected during the subsequent. oxidation-reduction cycles.

Other principal methods employed for the recovery of plutonium from solutions containing fission products involved merely removing that element by means of a suit able carrier, dissolving the resulting carrier product precipitate, reprecipitating the plutonium in the same manner, and continuing this procedure until the plutonium has been concentrated with respect to fission products to the desired extent, i.e., decontaminated to the desired extent. The chief disadvantage of such a process lies in the fact that in order to precipitate plutonium quantitatively, it is also necessary to precipitate the carrier cation substantially quantitatively, and hence a decrease in the proportion of carrier to plutonium is clearly not effected.

It has now been discovered that the disadvantages of the foregoing procedures can be readily overcome by concentrating plutonium, particularly in the tetravalent state, in accordance with the process of the present invention. By this process it has been found possible to concentrate and recover plutonium from dilute solutions thereof through the use of greatly reduced carrier-plutonium ratios. Broadly, the procedure of this invention involves first contacting a plutonium-containing solution with a suitable carrier therefor and thereafter removing and dis.- solving the resulting carrier product precipitate. To the solution thus obtained is then added a source of anions capable of combining with the carrier cation to form an insoluble compound in an amount such that the quantity of said insoluble carrier so formed is not substantially in excess of about 50% of the original carrier product precipitate. As a result of the formation of this limited amount of carrier precipitate, however, plutonium is removed from solution in amounts of the order of and above, of that originally present thereby effecting a marked decrease in the proportion of carrier to plutonium. To obtain further concentration of the plutonium and reduction in the bulk of carrier, the above-mentioned cycle may be repeated as many times as is considered necessary or desirable. When practicing the present invention, the carrier bulk may be reduced as much as fiftyfold in one step with accompanying carrying of approximately 90% of the plutonium originally in solution. In general, it may be said that carrier concentrations of the order of 50% of those employed in obtaining the initial product precipitate will be found satisfactory. In this connection it should be pointed out that while carrier concentration in substantially any proportion below the carrier mass of the original product precipitate may be utilized ,to effect a reduction in carrier bulk, it has been found that substantially equally good results in regard to the percentage of plutonium carried is achieved with carrier concentrations from between about 2 and of the original carrier mass. Therefore, from the standpoint of ease of operation as well as for economic considerations, the aforesaid range is considered preferable for carrying out of the process of this invention.

The carriers that may be utilized in eiiecting the process of the present invention constitute a rather large group of substances and are limited only to those compounds which do not carry plutonium by isomorphous substitution. Examples of suitable carriers having such properties are the fluorides, oxalates, and phosphates of trivalent bismuth, yttrium, lanthanum, and praseodymium, neodymium, and samarium.

While the process of the present invention may be utilized in concentrating the plutonium contained in substantially any type of solution, it is considered particularly suitable for the recovery and concentration of that element from solutions of neutron irradiated uranium, such as for example, those solutions obtained by dissolving neutron irradiated uranium in nitric acid. Prior to the removal of plutonium from such solutions, however, it is preferable to introduce nitric oxide or a mixture of nitrogen oxides in order to effect more complete separation of plutonium therefrom. A detailed description in regard to the method of obtaining this particular result is shown and claimed in copending application U.S. Serial No. 578,793, filed February 19, 1945, by George E. Moore.

Also in accordance with this invention, it has been found that if only 2% of the carrier is precipitated this small quantity of precipitate will carry approximately 90% of the plutonium giving a fiftyfold volume reduction for this step. However, by separating the initial precipitate and by applying a subsequent 2% precipitate to the same solution, an additional quantity of the plutonium is carried down aggregating a product recovery of the order of 99% for the two precipitates. If desired, the two precipitations may be accomplished in one centrifuge and in one tank. For example, the first 2% precipitation can be effected in the tank and the solution then transferred to the centrifuge wherein the first precipitate is separated. The eflluent liquor from the centrifuge is returned to the same tank, the second precipitation effected, and the resulting liquid carrying the second precipitate passed through the same centrifuge. This double precipitation effects a volume reduction by a factor of approximately 25 and the resulting precipitate carries about 99% of the plutonium originally present.

The process of the present invention may be further illustrated by the example which follows wherein a synthetic solution of plutonium containing a specified amount of active lanthanum is utilized in order to determine the exact quantity of lanthanum carrier precipitated in any given instant.

EXAMPLE I To a solution 1 normal in nitric acid and which contained 325 gamma counts per minute per cc. of radioactive La+ tracer and 112,500 alpha counts per minute per cc. of Pu+ tracer were added inactive La' ions in a concentration of 12.5 mg. per cc. A saturated solution of potassium fluoride was next added thereto and the resulting mixture agitated for a period of about two hours, after which the precipitate of lanthanum fluoride, carrying plutonium thereon, was isolated by centrifugation and the residue thus obtained washed with a saturated lanthanum fluoride solution. The washed precipitate was then analyzed for both gamma and alpha activity, thereby determining the percentage of plutonium carried by a given quantity of lanthanum fluoride. The conditions of operation employed in carrying out the above-mentioned process are set out in the table below:

Table Volume of Volume of Tnm h n m Active Potassium Fluoride Plutonium Run N0. Solution, Fluoride, Precipicarried, cc. cc, rated, percent percent EXAMPLE II Through one liter of a 10% uranyl nitrate solution prepared by dissolving neutron bombarded uranium metal in nitric acid was introduced nitric, oxide at a temperature of about 75 C. for one hour. Thereafter 15 g. of lanthanum nitrate was dissolved therein, and to the resulting solution was added suflicient hydrofluoric acid to cause substantially complete precipitation of the lanthanum in the form of its fluoride. The precipitate thus obtained, and which contained substantially allof the plutonium originally present, was next treated with an aqueous solution of a 50-50 mixture of potassium hydroxide and potassium carbonate for about an hour at C. whereby the lanthanum fluoride precipitate was metathesized to lanthanum hydroxide. The latter was next dissolved in 60% nitric acid and thereafter sufiicient potassium fluoride was added to precipitate lanthanum fluoride in an amount corresponding to approximately 2% of the mass of the initial product precipitate. This precipitate was separated by centrifugation and the eflluent liquor treated with an additional quantity of precipitate of lanthanum fluoride, asdescribed above, corresponding to about 2%. The precipitate produced was then separated in the same centrifuge in which thefirst precipitate was contained. On analysis it was found that the combined precipitates contained approximately 99% of the plutonium originally present in solution.

It is to be strictly understood that the foregoing examples are merely illustrative of the present invention and is not in any sense to be considered as limitative since it will be readily apparent to those skilled in the art that this invention is susceptible of numerous improvements as modifications Without departing from the scope thereof. For example, the principles set out in the above discussion can be applied to plutonium-containing solutions in which the plutonium present therein has a valence other than 4 provided the cation of the carrier employed does not have the same valence. In general, it may be said that any such improvements of modifications are to be regarded as lying within the scope of this invention.

I claim:

1. In a process for the recovery of plutonium from a dilute aqueous solution thereof by means of carrier precipitation involving precipitating in the solution carrier compound selected from the group consisting of fluorides, oxalates, and phosphates of trivalent bismuth, yttrium, lanthanum, praseodymium, neodymium, and Samarium, and then dissolving the precipitated carrier compound, along with the plutonium associated therewith, to form an aqueous solution thereof, the method of recovering plutonium from the resulting solution by means of carrier precipitation which comprises precipitating from the resulting solution, as a compound with one of said anions, a portion of the carrier cation present, with the amount of carrier compound thus precipitated corresponding to between 2 percent and 10 percent of the mass of carrier compound thrown down in the preceding carrier precipitation. V

2. In a process for the recovery of plutonium from a dilute aqueous solution thereof by means of a carrier precipitation of tetravalent plutonium effected by precipitating lanthanum fluoride in the solution, then dissolving the precipitated lanthanum fluoride, along with the plutonium associated therewith, to form an aqueous solution thereof, and then removing a predominant portion of contained plutonium from the resulting solution by means of carrier precipitation of tetravalent plutonium effected by precipitating, as lanthanum fluoride, lanthanum existing in the solution, the method of obtaining a carrier precipitate in the second said carrier precipitation having a plutonium to carrier ratio greater than that in the first said carrier precipitate, which comprises limiting the amount of carrier cation precipitated during the second carrier precipitation to an amount corresponding to not more than 50 percent of the total amount of carrier cation present in the solution before initiating the precipitation.

3. In a process for the recovery of plutonium from a dilute aqueous solution thereof by means of carrier precipitation effected by precipitaing lanthanum fluoride in the solution, then dissolving the precipitated lanthnum fluoride, along with the plutonium associated therewith, to form an aqueous solution thereof, and then removing a predominant portion of contained plutonium from the resulting solution by means of carrier precipitation effected by precipitating, as lanthanum fluoride, lanthanum existing in the solution, the method of obtaining a carrier precipitate in the second said carrier precipitation having a plutonium to carrier ratio greater than that in the first said carrier precipitate, which comprises limiting the amount of carrier cation precipitated during the second carrier precipitation to an amount corresponding to between 2 percent and percent of the total amount of carrier cation present in the solution before initiating precipitation.

4. In a process for the recovery of plutonium from a dilute aqueous solution thereof by means of carrier precipitation involving precipitating in the solution a carrier compound selected from the group consisting of fluorides, oxalates, and phosphates of trivalent bismuth, yttrium, lanthanum, praseodymium, neodymium, and samarium, and then dissolving the precipitated carrier compound, along with the plutonium associated therewith, to form an aqueous solution thereof, the method of recovering plutonium from the resulting solution by means of carrier precipitation which comprises precipitating from the solution, as a compound with one of the said anions, a

portion of the carrier cation present, with the amount of carrier compound thus precipitated corresponding to be- 5 tween 2 percent and 10 percent of the mass of carrier compound thrown down in the preceding carrier precipitation, separating this carrier precipitate from its supernatant solution, then precipitating a further small fraction of the carrier cation in the supernatant solution, as a compound with one of the said anions, thereby carrying down a large part of the residual plutonium therein.

5. In a process for the recovery of plutonium from a dilute aqueous solution thereof by means of carrier precipitation of tetravalent plutonium involving precipitating lanthanum fluoride in the solution, converting the pre cipitated lanthanum fluoride to a soluble form by slurrying the precipitated lanthanum fluoride with an aqueous solution of potassium hydroxide and potassium carbonate, and dissolving the carrier precipitate, thusly converted to lanthanum hydroxide, along with plutonium associated therewith, the method of removing plutonium from the resulting solution by means of carrier precipitation of tetravalent plutonium which comprises precipitating from the solution, as lanthanum fluoride, a portion of the lanthanum present, with the amount of lanthanum fluoride thus precipitated corresponding to between 2 percent and 10 percent of the mass of carrier compound thrown down in the preceding carrier precipitation, separating this carrier precipitate from its supernatant solution, then precipitating a further small fraction of the lanthanum in the supernatant solution, as lanthanum fluoride, thereby carrying down of a large part of the residual plutonium therein.

References Cited in the file of this patent OTHER REFERENCES McMillan et a1.: Radioactive Element 93, Physical Review, vol. 59, pages 1185-1186 (1940).

Seaborg et al.: MDDC-305, U.S.A.E.C. Document is-,

sued April 13, 1942, declassified August 28, 1946. 

1. IN A PROCESS FOR THE RECOVERY OF PLUTONIUM FROM A DILUTE AQUEOUS SOLUTION THEREOF BY MEANS OF CARRIER PRECIPITATION INVOLVING PRECIPATING IN THE SOLUTION CARRIER COMPOUNDED SELECTED FROM THE GROUP CONSISTING OF FLUORIDES, OXALATES, AND PHOSPHATES OF TRIVALENT BISMUTH, YTTRIUM, LANTHANUM, PRASEODYMIUM, NEODYMIUN, AND SAMARIUM, AND THEN DISSOLVING THE PRECIPATED CARRIER COMPOUND, ALONG WITH THE PLUTONIUM ASSOCAITED THEREWITH, TO FORM AN AQUEOUS SOLUTION THEREOF, THE METHOD OF RECOVERING PLUTONIUM FROM THE RESULTING SOLUTION BY MEANS OF CARRIER PRECCIPATION WHICH COMPRISES CPRCIPATING FROM THE RESULTING SOLUTION, AS A COMPOUND WITH ONE OF SAID ANOINS, A PORTION OF THE CARRIER CATION PRESENT, WITH THE AMOUNT OF CARRIER COMPOUND THUS PRECIPITATED CORRESPONDING TO BETWEEN 2 PERCENT AND 10 PERCENT OF THE MASS OF CARRIER COMPOUND THROWN DOWN IN THE PRECEDING CARRIER PRECIPITATION. 